Evaporative fuel vapor emission control systems

ABSTRACT

An evaporative emission control canister system comprises an initial adsorbent volume having an effective incremental adsorption capacity at 25° C. of greater than 35 grams n-butane/L between vapor concentration of 5 vol % and 50 vol % n-butane, and at least one subsequent adsorbent volume having an effective incremental adsorption capacity at 25° C. of less than 35 grams n-butane/L between vapor concentration of 5 vol % and 50 vol % n-butane, an effective butane working capacity (BWC) of less than 3 g/dL, and a g-total BWC of between 2 grams and 6 grams. The evaporative emission control canister system has a two-day diurnal breathing loss (DBL) emissions of no more than 20 mg at no more than 210 liters of purge applied after the 40 g/hr butane loading step.

TECHNICAL FIELD

The present disclosure, in various embodiments, relates generally toevaporative emission control systems. More particularly, the presentdisclosure relates to evaporative fuel vapor emission control systems.

BACKGROUND

Evaporation of gasoline fuel from motor vehicle fuel systems is a majorpotential source of hydrocarbon air pollution. Such emissions can becontrolled by the canister systems that employ activated carbon toadsorb the fuel vapor emitted from the fuel systems. Under certain modesof engine operation, the adsorbed fuel vapor is periodically removedfrom the activated carbon by purging the canister systems with ambientair to desorb the fuel vapor from the activated carbon. The regeneratedcarbon is then ready to adsorb additional fuel vapor.

An increase in environmental concerns has continued to drive strictregulations of the hydrocarbon emissions from motor vehicles even whenthe vehicles are not operating. When a vehicle is parked in a warmenvironment during the daytime heating (i.e., diurnal heating), thetemperature in the fuel tank increases resulting in an increased vaporpressure in the fuel tank. Normally, to prevent the leaking of the fuelvapor from the vehicle into the atmosphere, the fuel tank is ventedthrough a conduit to a canister containing suitable fuel adsorbentmaterials that can temporarily adsorb the fuel vapor. The fuel vaporfrom the fuel tank enters the canister through a fuel vapor inlet of thecanister and diffuses into the adsorbent volume where it is adsorbed intemporary storage before being released to the atmosphere through a ventport of the canister. Once the engine is turned on, ambient air is drawninto the canister system through the vent port of the canister. Thepurge air flows through the adsorbent volume inside the canister anddesorbs the fuel vapor adsorbed on the adsorbent volume before enteringthe internal combustion engine through a fuel vapor purge conduit. Thepurge air does not desorb the entire fuel vapor adsorbed on theadsorbent volume, resulting in a residue hydrocarbon (“heel”) that maybe emitted to the atmosphere. In addition, that heel in localequilibrium with the gas phase also permits fuel vapors from the fueltank to migrate through the canister system as emissions. Such emissionstypically occur when a vehicle has been parked and subjected to diurnaltemperature changes over a period of several days, commonly called“diurnal breathing losses.” The California Low Emission VehicleRegulation makes it desirable for these diurnal breathing loss (DBL)emissions from the canister system to be below 10 mg (“PZEV”) for anumber of vehicles beginning with the 2003 model year and below 50 mg,typically below 20 mg, (“LEV-II”) for a larger number of vehiclesbeginning with the 2004 model year. Now the California Low EmissionVehicle Regulation (LEV-III) requires canister DBL emissions not toexceed 20 mg as per the Bleed Emissions Test Procedure (BETP) as writtenin the California Evaporative Emissions Standards and Test Proceduresfor 2001 and Subsequent Model Motor Vehicles, Mar. 22, 2012.

Several approaches have been reported to reduce the diurnal breathingloss (DBL) emissions. One approach is to significantly increase thevolume of purge gas to enhance desorption of the residue hydrocarbonheel from the adsorbent volume. This approach, however, has the drawbackof complicating management of the fuel/air mixture to the engine duringpurge step and tends to adversely affect tailpipe emissions. See U.S.Pat. No. 4,894,072.

Another approach is to design the canister to have a relatively lowcross-sectional area on the vent-side of the canister, either by theredesign of existing canister dimensions or by the installation of asupplemental vent-side canister of appropriate dimensions. This approachreduces the residual hydrocarbon heel by increasing the intensity ofpurge air. One drawback of such approach is that the relatively lowcross-sectional area imparts an excessive flow restriction to thecanister. See U.S. Pat. No. 5,957,114.

Another approach for increasing the purge efficiency is to heat thepurge air, or a portion of the adsorbent volume having adsorbed fuelvapor, or both. However, this approach increases the complexity ofcontrol system management and poses some safety concerns. See U.S. Pat.Nos. 6,098,601 and 6,279,548.

Another approach is to route the fuel vapor through an initial adsorbentvolume and then at least one subsequent adsorbent volume prior toventing to the atmosphere, wherein the initial adsorbent volume has ahigher adsorption capacity than the subsequent adsorbent volume. SeeU.S. Pat. No. RE38,844.

The regulations on diurnal breathing loss (DBL) emissions continue todrive new developments for improved evaporative emission controlsystems, especially when the level of purge air is low. Furthermore, thediurnal breathing loss (DBL) emissions may be more severe for a hybridvehicle that includes both an internal combustion engine and an electricmotor. In such hybrid vehicles, the internal combustion engine is turnedoff nearly half of the time during vehicle operation. Since the adsorbedfuel vapor on the adsorbents is purged only when the internal combustionengine is on, the adsorbents in the canister of a hybrid vehicle ispurged with fresh air less than half of the time compared toconventional vehicles. A hybrid vehicle generates nearly the same amountof evaporative fuel vapor as the conventional vehicles. The lower purgefrequency of the hybrid vehicle can be insufficient to clean the residuehydrocarbon heel from the adsorbents in the canister, resulting in highdiurnal breathing loss (DBL) emissions.

Accordingly, it is desirable to have an evaporative emission controlsystem with low diurnal breathing loss (DBL) emissions even when a lowlevel of purge air is used, or when the adsorbents in the canister arepurged less frequently such as in the case of hybrid vehicles, or both.Though a passive approach has been greatly desired, existing passiveapproaches still leave DBL emissions at levels that are many timesgreater than the 20 mg LEV-III requirement when only a fraction of thehistorically available purge is now available.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the evaporative emission controlcanister system according to one embodiment of the disclosure, whereinthe canister system has one canister;

FIG. 2 is a cross-sectional view of the evaporative emission controlcanister system according to one embodiment of the disclosure, whereinthe canister system has one canister;

FIG. 3 is a cross-sectional view of the evaporative emission controlcanister system according to one embodiment of the disclosure, whereinthe canister system has one canister;

FIG. 4 is a cross-sectional view of the evaporative emission controlcanister system according to one embodiment of the disclosure, whereinthe canister system has a main canister and a supplemental canister;

FIG. 5 is a cross-sectional view of the evaporative emission controlcanister system according to one embodiment of the disclosure, whereinthe canister system has a main canister and a supplemental canister;

FIG. 6 is a cross-sectional view of the evaporative emission controlcanister system according to one embodiment of the disclosure, whereinthe canister system has a main canister and a supplemental canister;

FIG. 7 is a cross-sectional view of the evaporative emission controlcanister system according to one embodiment of the disclosure, whereinthe canister system has a main canister and a supplemental canister;

FIG. 8 is a simplified schematic drawing of the apparatus used for thedetermination of the butane adsorption capacity; and

FIGS. 9-22 are simplified schematic drawings of the evaporative emissioncontrol canister systems according to some non-limiting embodiments ofpresent disclosure.

DESCRIPTION

The present disclosure now will be described more fully hereinafter, butnot all embodiments of the disclosure are shown. While the disclosurehas been described with reference to exemplary embodiments, it will beunderstood by those skilled in the art that various changes may be madeand equivalents may be substituted for elements thereof withoutdeparting from the scope of the disclosure. In addition, manymodifications may be made to adapt a particular structure or material tothe teachings of the disclosure without departing from the essentialscope thereof.

The drawings accompanying the application are for illustrative purposesonly. They are not intended to limit the embodiments of the presentapplication. Additionally, the drawings are not drawn to scale. Elementscommon between figures may retain the same numerical designation.

In a particular embodiment, an evaporative emission control canistersystem includes one or more canisters. The evaporative emission controlcanister system comprises an initial adsorbent volume having aneffective incremental adsorption capacity at 25° C. of greater than 35grams n-butane/L between vapor concentration of 5 vol % and 50 vol %n-butane; and at least one subsequent adsorbent volume having aneffective incremental adsorption capacity at 25° C. of less than 35grams n-butane/L between vapor concentration of 5 vol % and 50 vol %n-butane, an effective butane working capacity (BWC) of less than 3g/dL, and a g-total BWC of between 2 grams and 6 grams. The initialadsorbent volume and the at least one subsequent adsorbent volume arelocated within a single canister, or the initial adsorbent volume andthe at least one subsequent adsorbent volume are located in separatecanisters that are connected to permit sequential contact by fuel vapor.The evaporative emission control canister system has a two-day diurnalbreathing loss (DBL) emissions of no more than 20 mg at no more thanabout 210 liters of purge applied after the 40 g/hr butane loading step.

FIG. 13 show non-limiting examples of some embodiments of theevaporative emission control canister system wherein an initialadsorbent volume and subsequent adsorbent volume(s) are located within asingle canister. FIGS. 4-7 show non-limiting examples of the embodimentsof the evaporative emission control canister system that includes morethan one canister, wherein an initial adsorbent volume and at least onesubsequent adsorbent volume are located in separate canisters that areconnected to permit sequential contact by fuel vapor.

FIG. 1 illustrates one embodiment of the evaporative emission controlcanister system having an initial adsorbent volume and a subsequentadsorbent volume within a single canister. Canister system 100 includesa support screen 102, a dividing wall 103, a fuel vapor inlet 104 from afuel tank, a vent port 105 opening to an atmosphere, a purge outlet 106to an engine, an initial adsorbent volume 201, and a subsequentadsorbent volume 202.

When an engine is off, the fuel vapor from a fuel tank enters thecanister system 100 through the fuel vapor inlet 104. The fuel vapordiffuses into the initial adsorbent volume 201, and then the subsequentadsorbent volume 202 before being released to the atmosphere through thevent port 105 of the canister system. Once the engine is turned on,ambient air is drawn into the canister system 100 through the vent port105. The purge air flows through the subsequent adsorbent volume 202 andthen the initial adsorbent volume 201, and desorbs the fuel vaporadsorbed on the adsorbent volumes 202, 201 before entering an internalcombustion engine through the purge outlet 106.

The evaporative emission control canister system may include more thanone subsequent adsorbent volume. By way of non-limiting example, theevaporative emission control canister system 100 may include an initialadsorbent volume 201 and three subsequent adsorbent volumes 202, 203,204 within a single canister, as illustrated in FIG. 2.

Additionally, the evaporative emission control canister system mayinclude an empty volume within the canister. As used herein, the term“empty volume” refers to a volume not including any adsorbent. Suchvolume may comprise any non-adsorbent including, but not limited to, airgap, foam spacer, screen, or combinations thereof. In a non-limitingexample shown in FIG. 3, the evaporative emission control canistersystem 100 may include an initial adsorbent volume 201; three subsequentadsorbent volumes 202, 203, 204 within a single canister; and an emptyvolume 205 between the subsequent adsorbent volumes 203 and 204.

By way of non-limiting example, FIGS. 4-7 shows the embodiments of theevaporative emission control canister system wherein the canister systemincludes more than one canister. As illustrated in FIG. 4, the canistersystem 100 includes a main canister 101, a support screen 102, adividing wall 103, a fuel vapor inlet 104 from a fuel tank, a vent port105 opening to an atmosphere, a purge outlet 106 to an engine, aninitial adsorbent volume 201 in the main canister 101, subsequentadsorbent volumes 202, 203, 204 in the main canister 101, a supplementalcanister 300 that includes a subsequent adsorbent volume 301, and aconduit 107 connecting the main canister 101 to the supplementalcanister 300.

When the engine is off, the fuel vapor from a fuel tank enters thecanister system 100 through the fuel vapor inlet 104 into the maincanister 101. The fuel vapor diffuses through the initial adsorbentvolume 201 and then the subsequent adsorbent volumes (202, 203, and 204)in the main canister 101 before entering the supplemental canister 300via the conduit 107. The fuel vapor diffuses through the subsequentadsorbent volume 301 inside the supplemental canister 300 before beingreleased to the atmosphere through the vent port 105 of the canistersystem. Once the engine is turned on, ambient air is drawn into thecanister system 100 through the vent port 105. The purge air flowsthrough the subsequent adsorbent volume 301 in the supplemental canister300, the subsequent adsorbent volumes (204, 203, 202) in the maincanister 101, and then the initial adsorbent volume 201 in the maincanister 101, to desorb the fuel vapor adsorbed on the adsorbent volumes(301, 204, 203, 202, 201) before entering the internal combustion enginethrough the purge outlet 106.

Similar to the main canister, the supplemental canister of theevaporative emission control canister system may include more than onesubsequent adsorbent volume. By way of non-limiting example, thesupplemental canister 300 of the evaporative emission control canistersystem 100 may include subsequent adsorbent volumes 301 and 302, asillustrated in FIG. 5.

Furthermore, the supplemental canister of the evaporative emissioncontrol canister system may include an empty volume between thesubsequent adsorbent volumes. By way of non-limiting example, thesupplemental canister 300 of the evaporative emission control canistersystem 100 may include subsequent adsorbent volumes (301, 302, and 303)and an empty volume 304 between the subsequent adsorbent volumes 302 and303 as illustrated in FIG. 6. In a non-limiting example shown in FIG. 7,the supplemental canister 300 of the evaporative emission controlcanister system 100 may include subsequent adsorbent volumes (301, 302,303), an empty volumes 304 between the subsequent adsorbent volumes 301and 302, and an empty volumes 305 between the subsequent adsorbentvolumes 302 and 303. As previously discussed, the term “empty volume”refers to a volume not including any adsorbent. Such volume may compriseany non-adsorbent including, but not limited to, air gap, foam spacer,screen, conduit, or combinations thereof.

Additionally, the evaporative emission control canister system mayinclude an empty volume between the main canister and the supplementalcanister.

When desired, the evaporative emission control canister system mayinclude more than one supplemental canister. The evaporative emissioncontrol canister system may further include one or more empty volumesbetween the main canister and a first supplemental canister, between thesupplement canisters, and/or at the end of the last supplementalcanister. By way of non-limiting example, the evaporative emissioncontrol canister system may include a main canister, a firstsupplemental canister, a second supplemental canister, a thirdsupplemental canister, an empty volume between the main canister and afirst supplemental canister, an empty volume between the first andsecond supplemental canister, and an empty volume at the end of thethird supplemental canister.

As discussed above, FIGS. 1-7 are merely exemplary embodiments of thedisclosed evaporative emission control canister system, and thoseskilled in the art may envision additional embodiments without departingfrom the scope of the present disclosure.

When desired, the total adsorbent volume (i.e., the sum of the initialadsorbent volume and the subsequent adsorbent volumes) may be the sameas the volume of the evaporative emission control canister system.Alternatively, the total adsorbent volume may be less than the volume ofthe evaporative emission control canister system.

In a particular embodiment, a method of reducing fuel vapor emissions inan evaporative emission control system comprises contacting the fuelvapor with an initial adsorbent volume having an effective incrementaladsorption capacity at 25° C. of greater than 35 grams n-butane/Lbetween vapor concentration of 5 vol % and 50 vol % n-butane, and withat least one subsequent adsorbent volume having an effective incrementaladsorption capacity at 25° C. of less than 35 grams n-butane/L betweenvapor concentration of 5 vol % and 50 vol % n-butane, an effectivebutane working capacity (BWC) of less than 3 g/dL, and a g-total BWC ofbetween 2 grams and 6 grams. The initial adsorbent volume and the atleast one subsequent adsorbent volume are located within a singlecanister, or the initial adsorbent volume and the at least onesubsequent adsorbent volume are located in separate canisters that areconnected to permit sequential contact by fuel vapor. The method ofreducing fuel vapor emissions has a two-day diurnal breathing loss (DBL)emissions of no more than 20 mg at no more than about 210 liters ofpurge applied after the 40 g/hr butane loading step.

The term “adsorbent component” or “adsorbent volume,” as used herein,refers to an adsorbent material or adsorbent containing material alongvapor flow path, and may consist of a bed of particulate material, amonolith, honeycomb, sheet or other material.

The term “nominal volume,” as used herein, refers to a sum of thevolumes of the adsorbent components, and does not include the volumes ofgaps, voids, ducts, conduits, tubing, plenum spaces or other volumesalong lengths of the vapor flow path that are devoid of adsorbentmaterial across the plane perpendicular to vapor flow path. For example,in FIG. 1 the total nominal volume of the canister system is the sum ofthe volumes of adsorbent volumes 201 and 202. For example, in FIGS. 2and 3, the total nominal volume of the canister system is the sum of thevolumes of adsorbent volumes 201, 202, 203, and 204. In FIG. 4, thetotal nominal volume of the canister system is the sum of the volumes ofadsorbent volumes 201, 202, 203, 204, and 301. In FIG. 5, the totalnominal volume of the canister system is the sum of the volumes ofadsorbent volumes 201, 202, 203, 204, 301, and 302. In FIGS. 6 and 7,the total nominal volume of the canister system is the sum of thevolumes of adsorbent volumes 201, 202, 203, 204, 301, 302, and 303.

Determination of Nominal Volume Apparent Density

The term “nominal volume apparent density,” as used herein, is the massof the representative adsorbent in the adsorbent volume divided by thenominal volume of adsorbent, where the length of the volume is definedas the in situ distance within the canister system between theperpendicular plane of the vapor flow path initially in contact with theadsorbent component and the perpendicular plan of the vapor flow pathexiting the adsorbent component.

Non-limiting examples of how to calculate the nominal volume apparentdensity for various forms of adsorbents are described herein.

(A) Granular, Pelletized, or Spherical Adsorbents of Uniform AdsorptiveCapacity Across the Length of the Adsorbent Component Flow Path

The standard method ASTM D 2854 (hereinafter “the Standard Method”) maybe used to determine the nominal volume apparent density of particulateadsorbents, such as granular and pelletized adsorbents of the size andshape typically used for evaporative emission control for fuel systems.The Standard Method may be used to determine the apparent density ofadsorbent volume, when it provides the same apparent density value asthe ratio of the mass and the nominal volume of the adsorbent bed foundin the canister system. The mass of the adsorbent by the Standard Methodis of the representative adsorbent used in the incremental adsorptionanalysis, i.e., equivalently including or excluding inert binders,fillers, and structural components within the adsorbent volume dependingon what representative material is analyzed as the adsorbent sample.

Furthermore, the nominal volume apparent density of adsorbent volume maybe determined using an alternative apparent density method, as definedbelow. The alternative method may be applied to nominal adsorbentvolumes that have apparent densities that are not comparably or suitablymeasured by the Standard Method. Additionally, the alternative apparentdensity method may be applied to particulate adsorbents in lieu of theStandard Method, due to its universal applicability. The alternativemethod may be applied to the adsorbent volume that may containparticulate adsorbents, non-particulate adsorbents, and adsorbents ofany form augmented by spacers, voids, voidage additives within a volumeor sequential similar adsorbent volumes for the effect of net reducedincremental volumetric capacity.

In the alternative apparent density method, the apparent density ofadsorbent volume is obtained by dividing the mass of adsorbent by thevolume of adsorbent, wherein:

(1) the dry mass basis of the representative adsorbent in the adsorbentvolume is measured. For example, a 0.200 g representative sample of the25.0 g total adsorbent mass in an adsorbent volume is measured foradsorptive capacity by the McBain method. Whereas the McBain methodyields an adsorption value of g-butane per g-adsorbent, the applicablemass is 25.0 g for the numerator in the apparent density of theadsorbent volume that then allows conversion of the McBain analyticalvalue to the volumetric property of the adsorbent volume; and

(2) the volume of the adsorbent component in the denominator of theapparent density is defined as the in situ geometric volume under whichthe superficial vapor flow path occurs within the canister system. Thelength of the volume is bounded by a plane perpendicular to thesuperficial vapor flow entrance of the adsorbent volume in question(i.e., the point at which there is adsorbent present on theperpendicular plane) and a plane perpendicular to the superficial flowat the vapor flow exit of the adsorbent volume in question (i.e., thepoint at which there is no adsorbent across the plane perpendicular tovapor flow).

(B) Honeycombs, Monolith, or Foam Adsorbents

(1) Cylindrical Honeycomb Adsorbents

The apparent density of cylindrical honeycomb absorbents may bedetermined according to the procedure of Purification Cellutions, LLC(Waynesboro, Ga.) SOP 500-115. The volume of adsorbent is a multiple ofthe cross-sectional area (A) and the length (h) of the adsorbent. Thelength (h) of the adsorbent is defined as the distance between the frontplane of the adsorbent perpendicular to vapor or gas flow entering theadsorbent and the back plane of the adsorbent where the vapor or gasexits the adsorbent. The volume measurement is that of the nominalvolume, which is also used for defining bed volume ratios for purge. Inthe case of a cylindrical honeycomb adsorbent of circular cross-section,the adsorbent cross-sectional area is determined by πd²/4, where d isthe average diameter measured at four points on each end of thehoneycomb. The nominal adsorbent volume and the nominal volume apparentdensity are calculated as follows:

Nominal Adsorbent Volume=h×A

Nominal Volume Apparent Density=Part Mass/(h×A)

wherein “Part Mass” is the mass of the adsorbent for which arepresentative adsorbent sample was tested for adsorptive properties,including representative proportions of inert or adsorptive binders andfillers.

By way of non-limiting examples, FIG. 9 shows the boundary definitionsfor the nominal volume of a honeycomb adsorbent 109 having across-sectional area A. The vapor or gas flows through the honeycombadsorbent 109 in the direction of D1 to D2. The vapor or gas enters thefront plane (F) of the adsorbent 109, flows through the length (h) ofthe adsorbent 109, and exits back plane (B) of the adsorbent 109. Thenominal volume of a honeycomb adsorbent 109 equals to thecross-sectional area A×the length h. Similarly, FIG. 10 shows theboundary definitions for the nominal volume of foam adsorbent 110.

(2) Pleated, Corrugated and Sheet Adsorbents

For pleated and corrugated adsorbents, the nominal adsorbent volumeincludes all the void space created by the pleats and corrugations. Thevolume measurement is that of the nominal volume, which is also used fordefining bed volume ratios for purge. The nominal volume and theapparent density of adsorbent are calculated as follows:

Nominal Adsorbent Volume=h×A

Nominal Volume Apparent Density=Part Mass/(h×A)

wherein

“Part Mass” is the mass of the adsorbent for which a representativeadsorbent sample was tested for adsorptive properties, includingrepresentative proportions of inert or adsorptive binders and fillers,

h is the length of adsorbent, defined as the distance between the frontplane of the adsorbent perpendicular to vapor or gas flow entering thefilter and the back plane of the adsorbent where the vapor or gas exitsthe filter, and

A is the cross-sectional area of adsorbent.

By way of non-limiting example, FIG. 11 shows the boundary definitionsfor the volume of a stacked corrugated sheet adsorbent monolith 111. Itis also within those skilled in the art to form such a monolith as anextruded honeycomb.

In the case of a pleated adsorbent, the adsorbent cross-sectional areais determined by L×W, where L is the distance from one edge of theadsorbent to the opposite edge of the adsorbent in direction X, and W isthe distance from one edge of the adsorbent to the opposite edge of theadsorbent in direction Y.

By way of non-limiting examples, FIG. 12 shows the boundary definitionsfor the volume of a single pleat or corrugation 112. FIG. 13 shows theboundary definitions for the volume of a pleated or corrugated sheet 113with vapor flow path provided through the sheet by some form ofpermeability to gas flow. The face of the sheet is perpendicular to thevapor flow. In contrast, FIG. 14 shows the boundary definitions for thevolume of a pleated or corrugated sheet 114 where its face is angled togas flow. FIG. 15 shows the boundary definitions for the volume of anadsorbent volume 115 of parallel adsorbent sheets. FIG. 16 shows theboundary definitions for the volume of an adsorbent sleeve 116.

Determination of Nominal Incremental Adsorption Capacity

The term “nominal incremental adsorption capacity,” as used herein,refers to an adsorption capacity according to the following equation:

Nominal Incremental Adsorption Capacity=[Adsorbed Butane at 50 vol%−Adsorbed Butane at 5 vol %]×Nominal Volume Apparent Density×1000

wherein

“Adsorbed Butane at 50 vol %” is the gram mass of absorbed n-butane pergram mass of adsorbent sample at 50 vol % butane concentration;

“Adsorbed Butane at 5 vol %” is the gram mass of absorbed n-butane pergram mass of adsorbent sample at 5 vol % butane concentration; and

“Nominal Volume Apparent Density” is as defined previously.

Determination of the Nominal Volume Butane Working Capacity (BWC)

The standard method ASTM D5228 may be used to determine the nominalvolume butane working capacity (BWC) of the adsorbent volumes containingparticulate granular and/or pelletized adsorbents.

A modified version of ASTM D5228 method may be used to determine thenominal volume butane working capacity (BWC) of the honeycomb, monolith,and/or sheet adsorbent volumes. The modified method may also be used forparticulate adsorbents, where the particulate adsorbents includefillers, voids, structural components, or additives. Furthermore, themodified method may be used where the particulate adsorbents are notcompatible with the standard method ASTM D5228, e.g., a representativeadsorbent sample may not be readily placed as the 16.7 mL fill in thesample tube of the test.

The modified version of ASTM D5228 method is as follows. The adsorbentsample is oven-dried for a minimum of eight hours at 110±5° C., and thenplaced in desiccators to cool down. The dry mass of the adsorbent sampleis recorded. The mass of the empty testing assembly is determined beforethe adsorbent sample is assembled into a testing assembly. Then, thetest assembly is installed into the a flow apparatus and loaded withn-butane gas for a minimum of 25 minutes (±0.2 min) at a butane flowrate of 500 ml/min at 25° C. and 1 atm pressure. The test assembly isthen removed from the BWC test apparatus. The mass of the test assemblyis measured and recorded to the nearest 0.001 grams. This n-butaneloading step is repeated for successive 5 minutes flow intervals untilconstant mass is achieved. For example, the total butane load time for a35 mm diameter×150 mm long honeycomb (EXAMPLE 2 Adsorbent 1) was 66minutes. The test assembly may be a holder for a honeycomb or monolithpart, for the cases where the nominal volume may be removed and testedintact. Alternatively, the nominal volume may need to be a section ofthe canister system, or a suitable reconstruction of the nominal volumewith the contents appropriately oriented to the gas flows, as otherwiseencountered in the canister system.

The test assembly is reinstalled to the test apparatus and purged with2.00 liter/min air at 25° C. and 1 atm pressure for a set selected purgetime (±0.2 min) according to the formula: Purge Time (min)=(719×NominalVolume (cc))/(2000 (cc/min)).

The direction of the air purge flow in the BWC test is in the samedirection as the purge flow to be applied in the canister system. Afterthe purge step, the test assembly is removed from the BWC testapparatus. The mass of the test assembly is measured and recorded to thenearest 0.001 grams within 15 minutes of test completion.

The nominal volume butane working capacity (BWC) of the adsorbent samplewas determined using the following equation:

${{Nominal}\mspace{14mu} {Volume}\mspace{14mu} B\; W\; C\mspace{14mu} \left( {g\text{/}{dL}} \right)} = \frac{{Amount}\mspace{14mu} {of}\mspace{14mu} {Butane}\mspace{14mu} {Purged}\mspace{14mu} (g)}{{Nominal}\mspace{14mu} {Adsorbent}\mspace{14mu} {Volume}\mspace{14mu} ({dL})}$

wherein

“Nominal Volume Apparent Density” is as defined previously, and

Amount of Butane Purged=Mass of the test assembly after loading−Mass ofthe test assembly after purge.

The term “g-total BWC,” as used herein, refers to g-amount of butanepurged.

Determination of Effective Volumetric Properties

The effective volume of adsorbents takes into account the air gaps,voids and other volumes between the nominal volumes of adsorbents alongthe vapor flow path that lack adsorbent. Thus, the effective volumetricproperties of adsorbent refer to the properties of the adsorbent thattake into account air gaps, voids and other volumes between the nominalvolumes of adsorbents that lack adsorbent along the vapor flow path.

The effective volume (V_(eff)) for a given length of the vapor flow pathis the sum of the nominal volumes of adsorbent (V_(nom, i)) presentalong that vapor path length plus adsorbent-free volumes along thatvapor flow path (V_(gap, j)).

V _(eff) =ΣV _(nom,i) +ΣV _(gap,j)

A volumetric adsorptive properties of an effective volume (B_(eff)),such as incremental adsorption capacity (g/L), apparent density (g/mL)and BWC (g/dL), is the sum of each property of the individual nominalvolumes to be considered as part of the effective volume (B_(nom, i))multiplied by each individual nominal volume (V_(nom, i)), then dividedby the total effective volume (V_(eff)):

B _(eff)=Σ(B _(nom,i) ×V _(nom,i))/V _(eff)

Thus, the term “effective incremental adsorption capacity” is the sum ofeach nominal incremental adsorption capacity multiplied by eachindividual nominal volume, and then divided by the total effectivevolume.

The term “effective butane working capacity (BWC)” is the sum of eachBWC value multiplied by each individual nominal volume, and then dividedby the total effective volume.

The term “effective apparent density” is the sum of each apparentdensity multiplied by each individual nominal volume, and then dividedby the total effective volume

The term “g-total BWC of the effective volume” is the sum of the g-totalBWC gram values of the nominal volumes within the effective volume.

As non-limiting examples of how to determine effective volume ofadsorbents, FIG. 17 shows the effective volume for three adsorbenthoneycomb nominal volumes connected in the flow path by gaps of equalcross-sectional areas, with the arrow in the direction of D1 to D2indicating vapor flow into the effective volume, towards the canistersystem vent. FIG. 18 shows three adsorbent honeycomb nominal volumesconnected by conduit sections of different cross-sectional areascompared with the honeycomb cross-sectional areas. In FIGS. 17 and 18,the honeycomb nominal volumes and the gaps appear symmetric. However, itis understood that the honeycomb nominal volumes and the gaps may havedifferent dimensions.

In some embodiments, the volumetric adsorptive properties of theadsorbent volumes may be deceased along the vapor flow path. By way ofnon-limiting example, the volumetric incremental capacity and butaneworking capacity (BWC) of the adsorbent volumes may be decreased towardsthe vent direction of the canister system. The diminished volumetricadsorptive properties may be attained by modifying the properties of theseparate sections of adsorbent, by varying the size of the gaps betweenadsorbent nominal volumes (FIG. 19), by adjusting the dimensions ofindividual adsorbent nominal volumes, separately (FIGS. 20 and 21), orby a combination thereof (FIG. 22). By way of non-limiting examples, asshown in FIGS. 20 and 21, the canister system (120, 121) may includeadsorbent volume sections “F,” “M,” and “B” along the flow path in thedirection of D1 to D2. The effective butane working capacities (BWC) ofthe adsorbent volume sections may be decreased along the flow path inthe direction of D1 to D2 (i.e., the effective BWC of the adsorbentvolume section F>the effective BWC of the adsorbent volume section M>theeffective BWC of the adsorbent volume section B). In some embodiments,the effective BWC of the adsorbent volume section M and/or section B maybe less than 3 g/dL, while the effective BWC of the canister system maybe more than or equal to 3 g/dl.

In a particular embodiment, the evaporative emission control systeminclude: a fuel tank for storing fuel; an engine having an air inductionsystem and adapted to consume the fuel; an evaporative emission controlcanister system comprising one or more canister(s); a fuel vapor inletconduit from the fuel tank to the canister system; a fuel vapor purgeconduit from the canister system to the air induction system of theengine; and a vent conduit for venting the canister system when theengine is off and for admission of purge air to the canister system whenthe engine is on. The evaporative emission control canister system isdefined by a fuel vapor flow path from the fuel vapor inlet conduit tothe initial adsorbent volume toward the at least one subsequentadsorbent volume and the vent conduit, and by an air flow path from thevent conduit to the at least one subsequent adsorbent volume toward theinitial adsorbent volume and the fuel vapor purge conduit. Theevaporative emission control canister system includes an initialadsorbent volume having an effective incremental adsorption capacity at25° C. of greater than 35 grams n-butane/L between vapor concentrationof 5 vol % and 50 vol % n-butane; and at least one subsequent adsorbentvolume having an effective incremental adsorption capacity at 25° C. ofless than 35 grams n-butane/L between vapor concentration of 5 vol % and50 vol % n-butane, an effective butane working capacity (BWC) of lessthan 3 g/dL, and a g-total BWC of between 2 grams and 6 grams. Theinitial adsorbent volume and the at least one subsequent adsorbentvolume are located within a single canister, or the initial adsorbentvolume and the at least one subsequent adsorbent volume are located inseparate canisters that are connected to permit sequential contact byfuel vapor. The evaporative emission control canister system has atwo-day diurnal breathing loss (DBL) emissions of no more than 20 mg atno more than about 210 liters of purge applied after the 40 g/hr butaneloading step.

The disclosed evaporative emission control system may provide lowdiurnal breathing loss (DBL) emissions even under a low purge condition.The evaporative emission performance of the disclosed evaporativeemission control system may be within the regulation limits defined bythe California Bleed Emissions Test Procedure (BETP), which is 20 mg orless, even under a low purge condition.

The term “low purge,” as used herein, refers to a purge level at orbelow 210 liters applied after the 40 g/hr butane loading step (i.e.,100 bed volumes for a 2.1 liter adsorbent component system).

The evaporative emission control system may provide low diurnalbreathing loss (DBL) emissions even when being purged at or below 210liters applied after the 40 g/hr butane loading step. In someembodiments, the evaporative emission control system may be purged at orbelow 157.5 liters applied after the 40 g/hr butane loading step.

The evaporative emission control system may provide low diurnalbreathing loss (DBL) emissions even when being purged at or below 100 BV(bed volumes based on a 2.1 liter nominal volume of the canister system)applied after the 40 g/hr butane loading step. In some embodiments, theevaporative emission control system may be purged at or below 75 BV(based on a 2.1 liter nominal volume of the canister system) appliedafter the 40 g/hr butane loading step.

In some embodiments, the evaporative emission control system may includea heat unit to further enhance the purge efficiency. By way ofnon-limiting example, the evaporative emission control system mayinclude a heat unit for heating the purge air, at least one subsequentadsorbent volume, or both.

The adsorbents suitable for use in the adsorbent volumes may be derivedfrom many different materials and in various forms. It may be a singlecomponent or a blend of different components. Furthermore, the adsorbent(either as a single component or a blend of different components) mayinclude a volumetric diluent. Non-limiting examples of the volumetricdiluents may include, but are not limited to, spacer, inert gap, foams,fibers, springs, or combinations thereof.

Any known adsorbent materials may be used including, but not limited to,activated carbon, carbon charcoal, zeolites, clays, porous polymers,porous alumina, porous silica, molecular sieves, kaolin, titania, ceria,or combinations thereof. Activated carbon may be derived from variouscarbon precursors. By way of non-limiting example, the carbon precursorsmay be wood, wood dust, wood flour, cotton linters, peat, coal, coconut,lignite, carbohydrates, petroleum pitch, petroleum coke, coal tar pitch,fruit pits, fruit stones, nut shells, nut pits, sawdust, palm,vegetables such as rice hull or straw, synthetic polymer, naturalpolymer, lignocellulosic material, or combinations thereof. Furthermore,activated carbon may be produced using a variety of processes including,but are not limited to, chemical activation, thermal activation, orcombinations thereof.

A variety of adsorbent forms may be used. Non-limiting examples of theadsorbent forms may include granular, pellet, spherical, honeycomb,monolith, pelletized cylindrical, particulate media of uniform shape,particulate media of non-uniform shape, structured media of extrudedform, structured media of wound form, structured media of folded form,structured media of pleated form, structured media of corrugated form,structured media of poured form, structured media of bonded form,non-wovens, wovens, sheet, paper, foam, or combinations thereof. Theadsorbent (either as a single component or a blend of differentcomponents) may include a volumetric diluent. Non-limiting examples ofthe volumetric diluents may include, but are not limited to, spacer,inert gap, foams, fibers, springs, or combinations thereof. Furthermore,the adsorbents may be extruded into special thin-walled cross-sectionalshapes, such as hollow-cylinder, star, twisted spiral, asterisk,configured ribbons, or other shapes within the technical capabilities ofthe art. In shaping, inorganic and/or organic binders may be used.

The honeycomb adsorbents may be in any geometrical shape including, butare not limited to, round, cylindrical, or square. Furthermore, thecells of honeycomb adsorbents may be of any geometry. Honeycombs ofuniform cross-sectional areas for the flow-through passages, such assquare honeycombs with square cross-sectional cells or spiral woundhoneycombs of corrugated form, may perform better than round honeycombswith square cross-sectional cells in a right angled matrix that providesadjacent passages with a range of cross-sectional areas and thereforepassages that are not equivalently purged. Without being bound by anytheory, it is believed that the more uniform cell cross-sectional areasacross the honeycomb faces, the more uniform flow distribution withinthe part during both adsorption and purge cycles, and, therefore, lowerDBL emissions from the canister system.

In some embodiments, the evaporative emission control system may furtherinclude one or more heat input unit(s) for heating one or more adsorbentvolume(s) and/or one or more empty volume(s). The heat input units mayinclude, but are not limited to, internal resistive elements, externalresistive elements, or heat input units associated with the adsorbent.The heat input unit associated with the adsorbent may be an elementseparate from the adsorbent (i.e., non-contacted with adsorbents).Alternatively, the heat input unit associated with the adsorbent may bea substrate or layer on to which the adsorbent is attached, bonded,non-bonded, or in physical contact. The heat input unit associated withthe adsorbent may be adsorbent directly heated electrically by havingappropriate resistivity. The resistivity properties of the adsorbent maybe modified by the addition of conductive or resistive additives andbinders in the original preparation of the adsorbent and/or in theforming of the adsorbent into particulate or monolithic forms. Theconductive component may be conductive adsorbents, conductivesubstrates, conductive additives and/or conductive binders. Theconductive material may be added in adsorbent preparation, added inintermediate shaping process, and/or added in adsorbent shaping intofinal form. Any mode of heat input unit may be used. By way ofnon-limiting example, the heat input unit may include a heat transferfluid, a heat exchanger, a heat conductive element, and positivetemperature coefficient materials. The heat input unit may or may not beuniform along the heated fluid path length (i.e., provide differentlocal intensities). Furthermore, the heat input unit may or may not bedistributed for greater intensity and duration of heating at differentpoints along the heated fluid path length.

Examples

Determination of Incremental Adsorption Capacity

FIG. 8 shows a simplified schematic drawing of the apparatus used forthe determination of the butane adsorption capacity. This is known inthe field as the McBain method. The apparatus 800 includes a sample pan801 and a spring 802 inside a sample tube 803, a rough vacuum pump 804,a diffusion pump 805, a stopcock 806, metal/O-ring vacuum valves807-809, a butane cylinder 810, a pressure readout unit 811, and atleast one conduit 812 connecting the components of the apparatus 800.

The representative adsorbent component sample (“adsorbent sample”) wasoven-dried for more than 3 hours at 110° C. before loading onto thesample pan 801 attached to the spring 802 inside the sample tube 803.Then, the sample tube 803 was installed into the apparatus 800. Theadsorbent sample shall include representative amounts of any inertbinders, fillers and structural components present in the nominal volumeof the adsorbent component when the Apparent Density value determinationequivalently includes the mass of the inert binders, fillers, andstructural components in its mass numerator. Conversely, the adsorbentsample shall exclude these inert binders, fillers, and structuralcomponents when the Apparent Density value equivalently excludes themass of the inert binders, fillers, and structural components in itsnumerator. The universal concept is to accurately define the adsorptiveproperties for butane on a volume basis within the nominal volume.

A vacuum of less than 1 torr was applied to the sample tube, and theadsorbent sample was heated at 105° C. for 1 hour. The mass of theadsorbent sample was then determined by the extension amount of thespring using a cathetometer. After that, the sample tube was immersed ina temperature-controlled water bath at 25° C. Air was pumped out of thesample tube until the pressure inside the sample tube was 10⁻⁴ torr.n-Butane was introduced into the sample tube until equilibrium wasreached at a selected pressure. The tests were performed for two datasets of four selected equilibrium pressures each, taken about 38 torrand taken about 380 torr. The concentration of n-butane was based on theequilibrium pressure inside the sample tube. After each test at theselected equilibrium pressure, the mass of the adsorbent sample wasmeasured based on the extension amount of the spring using cathetometer.The increased mass of the adsorbent sample was the amount of n-butaneadsorbed by the adsorbent sample. The mass of n-butane absorbed (ingram) per the mass of the adsorbent sample (in gram) was determined foreach test at different n-butane equilibrium pressures and plotted in agraph as a function of the concentration of n-butane (in % volume). A 5vol % n-butane concentration (in volume) at one atmosphere is providedby the equilibrium pressure inside the sample tube of 38 torr. A 50 vol% n-butane concentration at one atmosphere is provided by theequilibrium pressure inside the sample tube of 380 torr. Becauseequilibration at precisely 38 torr and 380 torr may not be readilyobtained, the mass of adsorbed n-butane per mass of the adsorbent sampleat 5 vol % n-butane concentration and at 50 vol % n-butane concentrationwere interpolated from the graph using the data points collected aboutthe target 38 and 380 torr pressures.

Alternatively, Micromeritics (such as Micromeritics ASAP 2020) may beused for determining the incremental butane adsorption capacity insteadof the McBain method.

Determination of Diurnal Breathing Loss (DBL) Emissions

The evaporative emission control systems of EXAMPLES 1-13 (identifiedbelow) were assembled with the selected amounts and types of adsorbentsas shown in TABLES 1-3.

Each example was uniformly preconditioned (aged) by repetitive cyclingof gasoline vapor adsorption using certified TF-1 fuel (9 RVP, 10 vol %ethanol) and 300 nominal bed volumes of dry air purge at 22.7 LPM basedon the main canister (e.g., 630 liters for a 2.1 L main canister and 450liters for a 1.5 L main canister). The gasoline vapor load rate was 40g/hr and the hydrocarbon composition was 50 vol %, generated by heatingtwo liters of gasoline to about 36° C. and bubbling air through at 200ml/min. The two-liter aliquot of fuel was replaced automatically withfresh gasoline every two hours until 5000 ppm breakthrough was detectedby a FID (flame ionization detector). A minimum of 25 aging cycles wereused on a virgin canister. The aging cycles were followed by a singlebutane adsorption/air purge step. This step was to load butane at 40g/hour at a 50 vol % concentration in air at one atm to 5000 ppmbreakthrough, soak for one hour, then purge with dry air for 21 minuteswith a total purge volume attained by selecting the appropriate constantair purge rate for that period. The canister was then soaked with theports sealed for 24 hour at 20° C.

The DBL emissions were subsequently generated by attaching the tank portof the example to a fuel tank filled 40 vol % (based on its ratedvolume) with CARB Phase II fuel (7 RVP, 0% ethanol). Prior toattachment, the filled fuel tank had been stabilized at 18.3° C. for 24hours while venting. The tank and the example were thentemperature-cycled per CARB's two-day temperature profile, each day from18.3° C. to 40.6° C. over 11 hours, then back down to 18.3° C. over 13hours. Emission samples were collected from the example vent at 5.5hours and 11 hours during the heat-up stage into Kynar bags. The Kynarbags were filled with nitrogen to a known total volume based on pressureand then evacuated into a FID to determine hydrocarbon concentration.The FID was calibrated with a 5000 ppm butane standard. From the Kynarbag volume, the emissions concentration, and assuming an ideal gas, themass of emissions (as butane) was calculated. For each day, the mass ofemissions at 5.5 hours and 11 hours were added. Following CARB'sprotocol the day with the highest total emissions was reported as “2-dayemissions.” In all cases, the highest emissions were on Day 2. Thisprocedure is generally described in SAE Technical Paper 2001-01-0733,titled “Impact and Control of Canister Bleed Emissions,” by R. S.Williams and C. R. Clontz, and in CARB's LEV III BETP procedure (sectionD.12 in California Evaporative Emissions Standards and Test Proceduresfor 2001 and Subsequent Model Motor Vehicles, Mar. 22, 2012).

For EXAMPLES 1-4, 13 and EXAMPLES 7-8, a 68 L fuel tank and a 2.1 litermain canister (TABLE 1, Main Canister Type #1) was used as a maincanister having fuel-source side volumes (i.e., an initial adsorbentvolume) filled with 1.8 liters of NUCHAR® BAX 1500 activated carbonadsorbent and a vent-side volume filled with 0.3 liters of NUCHAR® BAXLBE activated carbon adsorbent. The volumes were configured such thatthere was a 1500 ml fuel-source side chamber and a 600 ml vent-sidechamber, where the fuel-source chamber had a cross sectional area (CSA)that was 2.5 times the vent-side CSA. The BAX 1500 activated carbonfilled the fuel source chamber (similar to volumes 201 plus 202 in FIGS.2-7) and 300 mL of the immediate downstream volume in the vent-sidechamber (similar to volume 203 in FIGS. 2-7). The 300 mL of the BAX LBEactivated carbon filled the remaining volume of the vent-side chamber(similar to volume 204 in FIG. 7). NUCHAR® BAX 1500 activated andNUCHAR® BAX LBE activated carbon are wood-based activated carbonproducts, commercially available from MeadWestvaco Corporation, havingan incremental adsorption capacity at 25° C. of 80 grams n-butane/L and24 grams n-butane/L respectively, between vapor concentration of 5 vol %and 50 vol % n-butane (“Nominal Incremental Capacity” in TABLE 1). Forthe post-butane loading air purge step, each canister system in EXAMPLES1-4, 13 and EXAMPLES 7-8 was purged with 157.5 liters of purge air at apurge rate of 7.5 lpm. In terms of bed volume ratios of purge volumedivided by the total nominal volume of the canister systems, the purgeapplied was between 66.0 and 75.0 bed volumes (BV).

For EXAMPLES 5-6 and 9-12, a 45 L fuel tank and a 1.5 liter maincanister (TABLE 1, Main Canister Type #2) was used as a main canisterhaving fuel-source side volumes (i.e., an initial adsorbent volume)filled with 1.2 liters of NUCHAR® BAX 1100 activated carbon adsorbentand a vent-side volume filled with 0.3 liters of NUCHAR® BAX LBEactivated carbon adsorbent. The volumes were configured such that therewas a 1000 ml fuel-source side chamber and a 500 ml vent-side chamber,where the fuel-source chamber had a cross sectional area (CSA) that was2.0 times the vent-side CSA. The BAX 1100 activated carbon filled thefuel source chamber (similar to volumes 201 plus 202 in FIGS. 2-7) and200 mL of the immediate downstream volume in the vent-side chamber(similar to volume 203 in FIGS. 2-7). The 300 mL of the BAX LBEactivated carbon filled the remaining volume of the vent-side chamber(similar to volume 204 in FIG. 7). NUCHAR® BAX 1100 activated is awood-based activated carbon product, commercially available fromMeadWestvaco Corporation, having an incremental adsorption capacity at25° C. of 52 grams n-butane/L between vapor concentration of 5 vol % and50 vol % n-butane. During the post-butane loading air purge step, eachcanister system example was purged with either 100 or 150 liters ofpurge air at a purge rate of 4.76 or 7.14 lpm, respectively. In terms ofbed volume ratios of purge volume divided by the total nominal volume ofthe canister systems, the purge applied was between 55.9 and 91.2 BV.

EXAMPLES 1-13 each included none, one, or two additional vent-sideadsorbent volumes in-series. The first supplemental canister downstreamalong the vapor flow path from the main canister (if present) was notedas “Adsorbent 1” and a second in-series supplemental canister (ifpresent) downstream along the vapor flow path from Adsorbent 1 was notedas “Adsorbent 2.” One type of additional vent-side adsorbent (similar tosupplemental canister 300 in FIG. 4) was described as “35×150,” whichwas a 35 mm diameter×150 mm long, 200 cells per square inch (cpsi)cylindrical carbon honeycomb. The accounting of the effective volume forthe “35×150” adsorbent was the same boundaries as shown in FIG. 9, thatis, the effective volume was bounded by the vapor entrance and exitfaces of the honeycomb, and equal to its nominal volume. The second typeof additional vent-side adsorbent (similar to supplemental canister 300in FIG. 7) was described as “3-35×50,” which was three 35 mm diameter×50mm long, 200 cpsi cylindrical carbon honeycombs, including two 35 mmdiameter×7 mm thick foam spacers. Each foam spacer created a 7 mLvoidage gap between each sequential 50 mm long honeycomb length, similarto gaps 304 and 305 in FIG. 7. The accounting of the effective volumewas the same boundaries as shown in FIG. 17, that is, the effectivevolume was bounded by the vapor entrance face of the first of the threehoneycombs and exit faces of the third of the three honeycombs, andequal to the nominal volumes of the three honeycombs plus the volumes ofthe 7-mm thick spacers. The nominal incremental adsorption capacity at25° C. of n-butane/L between vapor concentration of 5 vol % and 50 vol %n-butane was shown as the “Nominal Incremental Capacity.” When based onthe effective volume, the incremental adsorption capacity at 25° C. ofn-butane/L between vapor concentration of 5 vol % and 50 vol % n-butanewas shown as the “Effective Incremental Capacity.” The two-day DBLemissions were reported as the “2-day DBL Emissions” in units of mg. Thereported results were often the average of several replicates of theBETP in order to verify findings.

The evaporative emission control canister system of EXAMPLES 1-4, 13 andEXAMPLES 7-8 each included an initial adsorbent volume of BAX 1500activated carbon adsorbent having a nominal incremental adsorptioncapacity at 25° C. of 80 g n-butane/L (i.e., more than 35 g/L) betweenvapor concentration of 5 vol % and 50 vol % n-butane, and a subsequentadsorbent volume of BAX LBE activated carbon adsorbent having a nominalincremental adsorption capacity at 25° C. of 24 g/L (less than 35 g/L)between vapor concentration of 5 vol % and 50 vol % n-butane (less than35 g/L). This is main canister type #1 in TABLE 1.

EXAMPLE 1 was the evaporative emission control canister system disclosedin the U.S. Pat. No. RE38,844. As shown in TABLE 2, the evaporativeemission control canister system of EXAMPLE 1 provided a 2-day DBLEmissions of 215 mg under a low purge condition of 75 bed volume (BV) ofpurge air after butane loading (i.e., 157.5 liters). These 2-day DBLEmissions were more than an order of magnitude above the 20 mgregulation limit under the California Bleed Emissions Test Procedure(BETP). Thus, the 20 mg regulation limits under the California BleedEmissions Test Procedure (BETP) could not be achieved by the evaporativeemission control canister system disclosed in the U.S. Pat. No.RE38,844.

For EXAMPLE 2, an additional vent-side adsorbent volume (Adsorbent 1)was added to EXAMPLE 1 in the form of an activated carbon honeycomb(“35×150”) having an effective incremental adsorption capacity at 25° C.of 16 g/L (less than 35 g/L) between vapor concentration of 5 vol % and50 vol % n-butane (less than 35 g/L), an effective BWC of 4.2 g/dL and ag-total BWC of 6.1 g. As shown in TABLE 2, the 2-day DBL Emissions forEXAMPLE 2 with a low purge level of 157.5 liters (applied after butaneloading) was 74 mg, which was still above the 20 mg regulation limitunder the California Bleed Emissions Test Procedure (BETP). Thus, at thepurge level of 157.5 liters applied after butane loading, theevaporative emission control canister system of the U.S. Pat. No.RE38,844 still could not satisfy the 20 mg regulation limit under BETPeven when it was used in combination with the additional vent-sideadsorbent volume (Adsorbent 1).

For EXAMPLE 3, a second additional vent-side adsorbent volume in theform of a activated carbon honeycomb (Adsorbent 2) of the same type andproperties as Adsorbent 1 (“35×150”) was added to the canister system ofEXAMPLE 2. Surprisingly, as shown in TABLE 2, there was only a marginalreduction in the 2-day DBL emissions from the additional vent-sideadsorbent volume in EXAMPLE 3, to 70 mg and still above the 20 mgregulation limit under the California Bleed Emissions Test Procedure(BETP).

EXAMPLE 4 was a variation of EXAMPLE 3 in that the activated carbonhoneycombs were each divided in to three 50 mm long section with narrowspacers in between. For EXAMPLE 4, the spacers reduced the effectiveincremental capacities of Adsorbents 1 and 2 to 14.6 g/L and reduced theeffective BWC to 3.9 g/dL, but, by definition, kept the g-total BWC thesame, at 6.1 g. As shown in TABLE 2, the 2-day DBL emissions of EXAMPLE4 remained high at 52 mg and were still above the 20 mg regulationlimits under the California Bleed Emissions Test Procedure (BETP).

In EXAMPLE 13, Adsorbent 2 was honeycombs divided into two 50 mm longsections with a narrow spacer in between. The effective incrementalcapacity was 6.1 g/L and the effective BWC was 1.6 g/dL. By definition,the g-total BWC was 1.6 g. As shown in TABLE 2, the 2-day DBL emissionsof EXAMPLE 13 remained high at 35 mg and were still above the 20 mgregulation limits under the California Bleed Emissions Test Procedure(BETP).

For EXAMPLE 7, Adsorbent 2 had an effective incremental capacity of 9.8g/L, an effective BWC of 2.6 g/dL and a g-total BWC of 4.0 g. ForEXAMPLE 8, Adsorbent 2 had an effective incremental capacity of 10.7g/L, an effective BWC of 2.8 g/dL and a g-total BWC of 4.4 g. As shownin TABLE 2, with 157.5 liters of purge, the canister systems of EXAMPLES7 and 8 provided the 2-day DBL Emissions of 10.3 g/dl and 13 g/dl,respectively. Thus, the canister systems of EXAMPLES 7 and 8 had the2-day DBL Emissions well below the BETP requirement of less than 20 mgfor low purge conditions of 157.5 liters (66.0 BV).

The evaporative emission control canister system of EXAMPLES 5, 6 and9-12 were based on the main canister type #2 in TABLE 1.

EXAMPLE 12 was the evaporative emission control canister system similarto those disclosed in the U.S. Pat. No. RE38,844. As shown in TABLE 3,the evaporative emission control canister system of EXAMPLE 12 did notinclude any additional adsorbent volume on the vent side. EXAMPLE 12provided 2-day DBL Emissions of 175 mg under a low purge condition of100 bed volume (BV) of purge air after butane loading (i.e., 150liters), which was about nine time higher than the 20 mg regulationlimit under the California Bleed Emissions Test Procedure (BETP). Thisconfirmed that the evaporative emission control canister system similarto those disclosed in the U.S. Pat. No. RE38,844 was not able to achievethe 2-day DBL Emissions requirements under the BETP (i.e., less than 20mg) when low purge was used.

In EXAMPLE 5, a low volume of purge after butane loading 150 liters wasapplied, or 91.2 BV for the 1.5 L nominal volume of the canister systemthat included an additional vent-side adsorbent volume of a “35×150”activated carbon honeycomb as Adsorbent 1. As shown in TABLE 3, the2-day DBL emissions were high at 57 mg and above the 20 mg regulationlimit under the California Bleed Emissions Test Procedure (BETP).

For EXAMPLE 6, the purge applied was reduced to 100 liters, or 55.9 BVfor the main canister type #2 that included the same additionalvent-side adsorbent volumes as EXAMPLE 4. As shown in TABLE 3, the 2-DayDBL emissions were high at 80 mg and above the 20 mg regulation limitsunder the California Bleed Emissions Test Procedure (BETP).

The canister systems of EXAMPLES 9, 10 and 11 each included an initialadsorbent volume of NUCHAR® BAX 1100 activated carbon adsorbent havingan incremental adsorption capacity at 25° C. of 52 g n-butane/L betweenvapor concentration of 5 vol % and 50 vol % n-butane (i.e., more than 35g/L) as part of the main canister type #2, and at least one subsequentadsorbent volume (“Adsorbent 2” in TABLE 3) having an effectiveincremental adsorption capacity at 25° C. butane adsorption capacity ofless than 35 g/L between vapor concentration of 5 vol % and 50 vol %n-butane and a g-total BWC of between 2 and 6 g.

Adsorbent 2 in EXAMPLE 9 had an effective incremental capacity of 11.7g/L, an effective BWC of 3.1 g/dL (greater than 3 g/dL) and a g-totalBWC of 4.8 g. As shown in TABLE 3, the 2-day DBL emissions for EXAMPLE 9under the low purge of 100 liters (i.e., 55.9 BV) were 51 mg and wellabove the BETP requirement of less than 20 mg.

In contrast, Adsorbent 2 in EXAMPLE 10 had an effective incrementalcapacity of 9.8 g/L, an effective BWC of 2.6 g/dL (less than 3 g/dL) anda g-total BWC of 4.0 g. As shown in TABLE 3, the 2-day DBL emissionsunder the low purge of 100 liters, equal to 55.9 BV, were 13.0 mg andwithin the BETP requirement of less than 20 mg.

Likewise, Adsorbent 2 in EXAMPLE 11 had an effective incrementalcapacity of 5.9 g/L, an effective BWC of 1.6 g/dL (less than 3 g/dL) anda g-total BWC of 2.4 g. As shown in TABLE 3, the 2-day DBL emissionsunder the low purge of 150 liters, equal to 83.9 BV, were 7.3 mg andwithin the BETP requirement of less than 20 mg.

TABLE 4 and TABLE 5 summarized the conditions of the canister systems ofEXAMPLES 1-13, and their measured 2-day DBL emissions. The canistersystems of EXAMPLES 7, 8, 10 and 11 provided the 2-day DBL emissions ofless than 20 mg, as required under the California Bleed Emissions TestProcedure (BETP). The requirement not to exceed 20 mg for BETP under lowpurge was met by satisfying a window of adsorptive properties by avent-side volume, where the window was an effective BWC of less than 3g/dL and a g-total BWC of between 2 g and 6 g. Thus, the means toachieve the BETP emissions requirement under low purge conditions wasmore than only a reduction in the working capacity or incrementalcapacity across the vapor flow path of the canister system andspecifically of the vent-side adsorbent volume to a prescribed level,but to additionally have sufficient gram working capacity in thatvent-side volume to restrain the emissions.

While the present disclosure is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, the present disclosure is not intended to be limited to theparticular forms disclosed. Rather, the present disclosure is to coverall modifications, equivalents, and alternatives falling within thescope of the present disclosure as defined by the following appendedclaims and their legal equivalents.

TABLE 1 Main Canister Type #1 #2 Fuel Side Nominal Volume (mL) 1800 1200Adsorbent Type BAX 1500 BAX 1100 Nominal Incremental Capacity (g/L) 8052 Nominal Apparent Density (g/mL) 0.284 0.358 Vent Side Nominal Volume(mL) 300 300 Adsorbent Type BAX LBE BAX LBE Nominal Incremental Capacity(g/L) 24 24 Nominal Apparent Density (g/mL) 0.383 0.383 Fuel Tank Size(rated L) 68 45

TABLE 2 EXAMPLE 1 2 3 4 13 7 8 Main Canister Type #1 #1 #1 #1 #1 #1 #1Additional Vent Side Adsorbents Volumes: Adsorbent 1 Nominal Volume (mL)None 144 144 144 144 144 144 Adsorbent 1 Type None “35 × 150” “35 × 150”“3-35 × 50” “3-35 × 50” “3-35 × 50” “3-35 × 50” Adsorbent 1 EffectiveVolume (mL) None 144 144 158 158 158 158 Nominal Incremental Capacity(g/L) — 16.0 16.0 16.0 16.0 16.0 16.0 Effective Incremental Capacity(g/L) — 16.0 16.0 14.6 14.6 14.6 14.6 Nominal Apparent Density (g/mL) —0.377 0.377 0.377 0.377 0.377 0.377 Effective Apparent Density (g/mL) —0.377 0.377 0.345 0.345 0.345 0.345 Nominal BWC (g/dL) — 4.2 4.2 4.2 4.24.2 4.2 Effective BWC (g/dL) — 4.2 4.2 3.8 3.8 3.8 3.8 g-Total BWC (g) —6.1 6.1 6.1 6.1 6.1 6.1 Adsorbent 2 Nominal Volume (mL) None None 144144 96 144 144 Adsorbent 2 Type None None “35 × 150” “3-35 × 50” “2-35 ×50” “3-35 × 50” “3-35 × 50” Adsorbent 2 Effective Volume (mL) None None144 158 103 158 158 Nominal Incremental Capacity (g/L) — — 16.0 16.0 6.510.7 11.7 Effective Incremental Capacity (g/L) — — 16.0 14.6 6.1 9.810.7 Nominal Apparent Density (g/mL) — — 0.377 0.377 0.559 0.493 0.487Effective Apparent Density (g/mL) — — 0.377 0.345 0.522 0.451 0.446Nominal BWC (g/dL) — — 4.2 4.2 1.7 2.8 3.1 Effective BWC (g/dL) — — 4.23.8 1.6 2.6 2.8 g-Total BWC (g) — — 6.1 6.1 1.6 4.0 4.4 Total NominalVolume of Canister 2.10 2.24 2.39 2.39 2.34 2.39 2.39 System (L) PurgeApplied After the 40 g/hr Butane 157.5 157.5 157.5 157.5 157.5 157.5157.5 Loading Step, liters Purge Applied After the 40 g/hr Butane 75.070.2 66.0 66.0 67.3 66.0 66.0 Loading Step, BV 2-Day DBL Emissions, mg215 74 70 52 35 10.3 13

TABLE 3 EXAMPLE 12 5 6 9 10 11 Main Canister Type #2 #2 #2 #2 #2 #2Additional Vent Side Adsorbents Volumes: Adsorbent 1 Nominal Volume (mL)None 144 144 144 144 144 Adsorbent 1 Type None “35 × 150” “3-35 × 50”“3-35 × 50” “3-35 × 50” “3-35 × 50” Adsorbent 1 Effective Volume (mL)None 144 158 158 158 158 Nominal Incremental Capacity (g/L) — 16.0 16.016.0 16.0 16.0 Effective Incremental Capacity (g/L) — 16.0 14.6 14.614.6 14.6 Nominal Apparent Density (g/mL) — 0.377 0.377 0.377 0.3770.377 Effective Apparent Density (g/mL) — 0.377 0.345 0.345 0.345 0.345Nominal BWC (g/dL) — 4.2 4.2 4.2 4.2 4.2 Effective BWC (g/dL) — 4.2 3.83.8 3.8 3.8 g-Total BWC (g) — 6.1 6.1 6.1 6.1 6.1 Adsorbent 2 NominalVolume (mL) None None 144 144 144 144 Adsorbent 2 Type None None “3-35 ×50” “3-35 × 50” “3-35 × 50” “3-35 × 50” Adsorbent 2 Effective Volume(mL) None None 158 158 158 158 Nominal Incremental Capacity (g/L) — —16.0 12.8 10.7 6.5 Effective Incremental Capacity (g/L) — — 14.6 11.79.8 5.9 Nominal Apparent Density (g/mL) — — 0.377 0.438 0.493 0.558Effective Apparent Density (g/mL) — — 0.345 0.399 0.451 0.511 NominalBWC (g/dL) — — 4.2 3.4 2.8 1.7 Effective BWC (g/dL) — — 3.8 3.1 2.6 1.6g-Total BWC (g) — — 6.1 4.8 4.0 2.4 Total Nominal Volume of CanisterSystem (L) 1.50 1.64 1.79 1.79 1.79 1.79 Purge Applied After the 40 g/hrButane 150 150 100 100 100 150 Loading Step, liters Purge Applied Afterthe 40 g/hr Butane 100 91.2 55.9 55.9 55.9 83.9 Loading Step, BV 2-DayDBL Emissions, mg 175 57 80 51 13 7.3

TABLE 4 EX. Main Canister Type #1 EX. 1 EX. 2 EX. 3 EX. 4 13 EX. 7 EX. 8Fuel side Effective Incremental 80 80 80 80 80 80 80 AdsorptionCapacity, g/L Vent Side Adsorbent Volume #0 Effective Incremental 24 2424 24 24 24 24 Adsorption Capacity, g/L Effective BWC, g/dL 6.3 6.3 6.36.3 6.3 6.3 6.3 g-Total BWC, g 18.9 18.9 18.9 18.9 18.9 18.9 18.9Adsorbent Volume #1 Effective Incremental N/A 16.0 16.0 16.0 14.6 14.614.6 Adsorption Capacity, g/L Effective BWC, g/dL N/A 4.2 4.2 4.2 4.23.8 3.8 g-Total BWC, g N/A 6.1 6.1 6.1 6.1 6.1 6.1 Adsorbent Volume #2Effective Incremental N/A N/A 16.0 14.6 6.1 7.3 8.0 Adsorption Capacity,g/L Effective BWC, g/dL N/A N/A 4.2 3.8 1.6 2.6 2.8 g-Total BWC, g N/AN/A 6.1 6.1 1.6 4.0 4.4 2-Day DBL Emission, 215 74 70 52 35 10.3 13 mg

TABLE 5 Main Canister Type #2 EX. 12 EX. 5 EX. 6 EX. 9 EX. 10 EX. 11Fuel side Effective Incremental 52 52 52 52 52 52 Adsorption Capacity,g/L Vent Side Adsorbent Volume #0 Effective Incremental 24 24 24 24 2424 Adsorption Capacity, g/L Effective BWC, g/dL 6.3 6.3 6.3 6.3 6.3 6.3g-Total BWC, g 18.9 18.9 18.9 18.9 18.9 18.9 Adsorbent Volume #1Effective Incremental N/A 16.0 14.6 14.6 14.6 14.6 Adsorption Capacity,g/L Effective BWC, g/dL N/A 4.2 3.8 3.8 3.8 3.8 g-Total BWC, g N/A 6.16.1 6.1 6.1 6.1 Adsorbent Volume #2 Effective Incremental N/A N/A 14.611.7 7.3 2.7 Adsorption Capacity, g/L Effective BWC, g/dL N/A N/A 3.83.1 2.6 1.6 g-Total BWC, g N/A N/A 6.1 4.8 4.0 2.4 2-Day DBL Emission,175 57 80 51 13 7.3 mg

1. An evaporative emission control canister system for a hybrid vehicle,including a hybrid vehicle power train and one or more canisters andcomprising: a fuel-side adsorbent volume having an effective incrementaladsorption capacity at 25° C. of greater than 35 grams n-butane/Lbetween vapor concentration of 5 vol % and 50 vol % n-butane; and atleast one subsequent adsorbent volume having an effective incrementaladsorption capacity at 25° C. of less than 35 grams n-butane/L betweenvapor concentration of 5 vol % and 50 vol % n-butane, a butane workingcapacity (BWC) of less than 3 g/dL, wherein the fuel-side adsorbentvolume having an effective incremental adsorption capacity at 25° C. ofgreater than 35 grams n-butane/L between vapor concentration of 5 vol %and 50 vol % n-butane, and the at least one subsequent adsorbent volumeare located within a single canister, or in separate canisters that areconnected to permit sequential contact by fuel vapor.
 2. The canistersystem of claim 1, wherein the canister system has a two-day diurnalbreathing loss (DBL) emissions of no more than 20 mg as determined bythe 2012 California Bleed Emissions Test Procedure (BETP).
 3. Thecanister system of claim 1, wherein the canister system has a two-daydiurnal breathing loss (DBL) emissions of at least one of: (i) no morethan 20 mg at no more than 210 liters of purge applied after a 40 g/hrbutane loading step, or (ii) no more than 20 mg at no more than 100 bedvolumes (BV) of purge applied after a 40 g/hr butane loading step. 4.The canister system of claim 1, wherein the at least one subsequentadsorbent volume has a g-total BWC of between 2 g and 6 g.
 5. Thecanister system of claim 1, wherein the at least one subsequentadsorbent volume has an effective incremental adsorption capacity at 25°C. of 24 or less grams n-butane/L between vapor concentration of 5 vol %and 50 vol % n-butane.
 6. The canister system of claim 1, wherein the atleast one subsequent adsorbent volume has an effective incrementaladsorption capacity at 25° C. of 16 or less grams n-butane/L betweenvapor concentration of 5 vol % and 50 vol % n-butane.
 7. The canistersystem of claim 1, further comprising at least one additional subsequentadsorbent volume having an effective incremental adsorption capacity at25° C. of less than 35 grams n-butane/L between vapor concentration of 5vol % and 50 vol % n-butane.
 8. The canister system of claim 7, whereinthe at least one additional subsequent adsorbent volume has a lowereffective incremental adsorption capacity relative to the adsorbentvolume that precedes it in the flow path from fuel-side to vent-side. 9.The canister system of claim 7, wherein each additional subsequentadsorbent volume has a lower effective incremental adsorption capacitythan any preceding adsorbent volume.
 10. The canister system of claim 7,wherein each additional subsequent adsorbent volume has a gram-total BWCof less than 6 g.
 11. The canister system of claim 1, wherein the systemfurther includes at least one heat input unit for heating one or moreadsorbent volumes.
 12. The canister system of claim 11, wherein the heatinput unit include at least one of an internal resistive element,external resistive element, or heat input unit associated with theadsorbent.
 13. The canister system of claim 12, wherein the heat inputunit includes at least one of a heat transfer fluid, a heat exchanger, aheat conductive element, or a positive temperature coefficient material.14. The canister system of claim 1, wherein the initial adsorbentvolume, subsequent volume or both includes an adsorbent selected fromthe group consisting of activated carbon, carbon charcoal, zeolites,clays, porous polymers, porous alumina, porous silica, molecular sieves,kaolin, titania, ceria, and combinations thereof.
 15. The canistersystem of claim 14, wherein the initial adsorbent volume, subsequentvolume or both includes an adsorbent selected from the group consistingof activated carbon, carbon charcoal, and combinations thereof.
 16. Thecanister system of claim 15, wherein the activated carbon is derivedfrom a material including a member selected from the group consisting ofwood, wood dust, wood flour, cotton linters, peat, coal, coconut,lignite, carbohydrates, petroleum pitch, petroleum coke, coal tar pitch,fruit pits, fruit stones, nut shells, nut pits, sawdust, palm,vegetables, synthetic polymer, natural polymer, lignocellulosicmaterial, and combinations thereof.
 17. The canister system of claim 16,wherein a form of adsorbent in the fuel-side adsorbent volume, the atleast one subsequent adsorbent volume, or both includes a memberselected from the group consisting of granular, pellet, spherical,honeycomb, monolith, pelletized cylindrical, particulate media ofuniform shape, particulate media of non-uniform shape, structured mediaof extruded form, structured media of wound form, structured media offolded form, structured media of pleated form, structured media ofcorrugated form, structured media of poured form, structured media ofbonded form, non-wovens, wovens, sheet, paper, foam, hollow-cylinder,star, twisted spiral, asterisk, configured ribbons, and combinationsthereof.
 18. The canister system of any of claim 1, wherein the at leastone subsequent adsorbent volume includes a volumetric diluent.
 19. Thecanister system of claim 18, wherein the volumetric diluent includes amember selected from the group consisting of inert spacer particles,trapped air spaces, foams, fibers, screens, and combinations thereof.20. The canister system of claim 19, wherein the volumetric diluentincludes an adsorbent material formed into a high voidage shape selectedfrom the group consisting of stars, hollow tubes, asterisks, spirals,cylinders, configured ribbons, honeycombs, monoliths, and combinationsthereof.
 21. An evaporative emission control canister system for ahybrid vehicle, including a hybrid vehicle power train and one or morecanisters and comprising: a fuel-side adsorbent volume having aneffective incremental adsorption capacity at 25° C. of greater than 35grams n-butane/L between vapor concentration of 5 vol % and 50 vol %n-butane; at least one subsequent adsorbent volume having an effectiveincremental adsorption capacity at 25° C. of less than 35 gramsn-butane/L between vapor concentration of 5 vol % and 50 vol % n-butane,a butane working capacity (BWC) of less than 3 g/dL; and at least oneheat input unit for heating one or more adsorbent volumes, wherein thefuel-side adsorbent volume having an effective incremental adsorptioncapacity at 25° C. of greater than 35 grams n-butane/L between vaporconcentration of 5 vol % and 50 vol % n-butane, and the at least onesubsequent adsorbent volume are located within a single canister, or inseparate canisters that are connected to permit sequential contact byfuel vapor.
 22. The canister system of claim 21, wherein the canistersystem has a two-day diurnal breathing loss (DBL) emissions of no morethan 20 mg as determined by the 2012 California Bleed Emissions TestProcedure (BETP).
 23. The canister system of claim 22, wherein thecanister system has a two-day diurnal breathing loss (DBL) emissions ofat least one of: (i) no more than 20 mg at no more than 210 liters ofpurge applied after a 40 g/hr butane loading step, or (ii) no more than20 mg at no more than 100 bed volumes (BV) of purge applied after a 40g/hr butane loading step.
 24. The canister system of claim 21, whereinthe at least one subsequent adsorbent volume has a g-total BWC ofbetween 2 g and 6 g.
 25. The canister system of claim 21, wherein the atleast one subsequent adsorbent volume has an effective incrementaladsorption capacity at 25° C. of 24 or less grams n-butane/L betweenvapor concentration of 5 vol % and 50 vol % n-butane.
 26. The canistersystem of claim 21, wherein the at least one subsequent adsorbent volumehas an effective incremental adsorption capacity at 25° C. of 16 or lessgrams n-butane/L between vapor concentration of 5 vol % and 50 vol %n-butane.
 27. The canister system of claim 21, further comprising atleast one additional subsequent adsorbent volume having an effectiveincremental adsorption capacity at 25° C. of less than 35 gramsn-butane/L between vapor concentration of 5 vol % and 50 vol % n-butane.28. The canister system of claim 27, wherein the at least one additionalsubsequent adsorbent volume has a lower effective incremental adsorptioncapacity relative to the adsorbent volume that precedes it in the flowpath from fuel-side to vent-side.
 29. The canister system of claim 27,wherein each additional subsequent adsorbent volume has a lowereffective incremental adsorption capacity than any preceding adsorbentvolume.
 30. The canister system of claim 27, wherein each additionalsubsequent adsorbent volume has a gram-total BWC of less than 6 g.